1. Field of the Invention
The present invention relates to a discharge tube and more particularly to a voltage control discharge tube used on laser equipment's starting gap switches, sharpener gaps and lightning arresters.
2. Description of the Prior Art
Among the discharge tubes that are used for laser equipment's starting gap switches, sharpener gaps and lightning arresters is one which includes two electrode plates disposed at both ends of an electrically insulating, cylindrical body in such a way as to face each other, with their discharge electrodes protruding inwardly. Such a discharge tube, however, has a drawback that because high voltages are impressed between the electrodes, the discharge characteristics easily vary because of influences from a grounded body present near the discharge tube.
To cope with this problem, an improvement has been proposed in Japanese Patent Preliminary Publication No. Heisei 3-141574, in which one of the electrodes of the discharge tube is enclosed by an enclosure, whose outer wall surface is provided with a conductive member to shield the electrode to maintain a stable discharge characteristic even when a grounded body comes near the discharge tube. This type of discharge tube, as shown in FIG. 3, has a conductive member 3 as a shield cover (hereinafter referred to simply as a shield) fitted to the outer wall surface of the enclosure 1 in such a way as to enclose the discharge electrode 2 to which a negative high voltage is applied, with the electrode plate 2a and the conductive member 3 interconnected.
With the discharge tube incorporating the above improvement, however, since the effective length L of the external wall surface of the enclosure decreases, the creeping discharge or flashover along the outer wall surface, indicated at D in the figure, becomes more likely to occur, so that it is necessary to elongate the effective length of the enclosure as shown by L1 in FIG. 4 to prevent the flashover. This reduces the degree of freedom of design because of restrictions in shape when incorporating the discharge tube into small devices. Further, the additional process of providing a conductive member to the enclosure increases the manufacture cost.